A hydrostatic-mechanical composite transmission is described, for example, in German Pat. No. 2 415 002. In this system, the mechanical transmission branch is formed as a gear-change transmission so that during direction change, all of the parts connected to the input shaft of the mechanical transmission, which has only a single output shaft, must be reversed in direction and the rotating masses of these parts must likewise change in rotational sense. In addition, a compensation of the throughput of the two hydrostatic units is required so that the hydrostatic system must be switched from a completely negative to a completely positive range. The terms "positive" and "negative" are here used to represent fluid displacement in opposite directions, i.e. reversal of the output port of the pump and the input port thereof so that the formerly output port becomes the input port and the previous input port becomes the output port.
A positive clutch is required in this system which can only come into play when both of the range-selecting clutches are open. For a reversible vehicle, such a transmission is not successful. The use of such a transmission results in high wear of the moving parts because of the operational limitations discussed above.